The invention relates to a diffractive security element as set forth in the classifying portion of claim 1.
Diffractive security elements of that kind are used for the verification of articles such as banknotes, passes and identity cards of all kinds, valuable documents and so forth in order to be able to establish the authenticity of the article without involving a high level of cost. When the article is issued the diffractive security element is fixedly joined thereto, in the form of a stamp portion cut from a thin layer composite.
Diffractive security elements of the kind set forth in the opening part of this specification are known from EP 0 105 099 A1 and EP 0 375 833 A1. Those security elements include a pattern of surface elements which are arranged in a mosaic-like fashion and which have a diffraction grating. The diffraction gratings are azimuthally predetermined in such a way that, upon a rotary movement, the visible pattern produced by diffracted light optically changes.
EP 0 360 969 A1 describes diffractive security elements in which the surface elements have asymmetrical diffraction gratings. The asymmetrical diffraction gratings are arranged in paired and mirror image symmetrical relationship in each two surface elements with a common boundary. Special asymmetrical diffraction gratings which act like inclinedly positioned mirrors are described in WO 97/19821.
The diffraction properties of the diffraction grating can be represented as an image on the basis of a Fourier space representation. That representation, in a circle, indicates the direction of the diffracted light beams by means of a point, the light being incident perpendicularly onto the diffraction grating at the center of the circle. The center of the circle corresponds to the diffraction angle β=0° and the periphery corresponds to the diffraction angle β=90°, while a radius at a point in the circle indicates the diffraction angle β of the light beams diffracted at the diffraction gratings. Polar angles of various points in the Fourier space representation reflect the azimuthal orientation of the diffraction gratings.
The diffractive security elements generally comprise a portion of a thin layer composite of plastic material. The interface between two of the layers has microscopically fine reliefs of light-diffracting structures. To enhance reflectivity, the interface between the two layers is covered with a reflection layer. The structure of the thin layer composite and the materials which can be used for that purpose are described for example in U.S. Pat. No. 4,856,857 and WO 99/47983. It is known from DE 33 08 831 A1 for the thin layer composite to be applied to the article by means of a carrier film.
The disadvantage of such diffractive security elements lies in the narrow solid angle and the extremely high level of surface brightness, at which a surface element covered with a diffraction grating is visible to an observer. The high level of surface brightness can also make it difficult to recognize the shape of the surface element.
It is also known from EP 0 712 012 A1 for microscopically fine stochastic roughness to be superimposed on a sinusoidal, submicroscopically fine diffraction grating, in such a way that the diffraction grating is stochastically modulated. The microscopically fine stochastic roughness is not further described and is produced by anisotropic process steps which cannot be reproduced, in manufacture of the master die. The submicroscopically fine diffraction grating alone, when directed light is involved, is visible only at the reflection angle. The roughness which is superimposed on the diffraction grating provides that the light diffracted at the submicroscopically fine diffraction grating is scattered into the half-space over the diffraction grating.